In mzscheme,
Welcome to MzScheme version 299.100, Copyright (c) 2004-2005 PLT Scheme,
Inc.
>
> (format "~10,0F" -3e-4)
"       -0."
> (format "~10,1F" -3e-4)
"      -0.0"
> (format "~10,2F" -3e-4)
"     -0.0."                    ---> error?
> (format "~10,3F" -3e-4)
"    -0.0.0"                    ---> error?
> (format "~10,4F" -3e-4)
"   -0.0003"

> 3e-5
3e-005
> (format "~10,4F"  3e-5)
> +: expects type <number> as 1st argument, given: #f; other arguments
were: 1

--
Joo ChurlSoo